I'm running into this as well.
My DNS prvoider is hard coded to 68.105.28.11 on my router/modem I rent from COX: https://whois.domaintools.com/68.105.28.11
This is a COX run DNS server. Every time a device on your network tries to resolve a domain name (i.e. your smart TV, laptop, etc) a request gets made to COX allowing them to derive what devices you have in your house (your smarttv will phone home to a roku domain, your xbox to a Microsoft domain, etc.), what websites you frequent, and so on.
I strongly suspect they are trying to monetize this data (build a profile on you about your behavior online and the devices in your home, then sell that profile to third parties). By locking down your DNS to their servers, it guarantees they have access to this data if you rent their hardware.
This seems very shady.